DC motors worksheet with answers

Free printable DC motors worksheet: 6 problems from easy to exam level, with an answer key. Print it or save it as a PDF.

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1.Easy

A DC motor on a 240 V supply has an armature resistance of 1 Ω. At its present speed the back-EMF is 225 V. Find the armature current I_a.

V = 240 VE = 225 VR_a = 1 Ω

I_a = A

2.Easy

A permanent-magnet DC motor has the torque constant k = 0.12 N·m/A. Find the torque T it develops at an armature current of 5 A.

k = 0.12 N·m/AI_a = 5 A

T = N·m

3.Medium

A DC motor draws 25 A from a 120 V supply. Its armature resistance is 0.25 Ω and its rotational (friction, windage and iron) losses are 142 W. Find its efficiency η.

V = 120 VI_a = 25 AR_a = 0.25 ΩP_rot = 142 W

η = %

4.Medium

A DC motor (k = 0.75 V·s/rad, R_a = 0.8 Ω) runs from 48 V. At full load it draws 2 A. Neglecting the no-load current, find its speed regulation SR = (n₀ − n_FL)/n_FL in percent.

V = 48 Vk = 0.75 V·s/radR_a = 0.8 ΩI_FL = 2 A

SR = %

5.Hard

A DC motor with k = 0.6 V·s/rad and R_a = 1 Ω must drive a load of 9 N·m at 750 rpm. Find the armature voltage V it needs.

k = 0.6 V·s/radR_a = 1 ΩT = 9 N·mn = 750 rpm

V = V

6.Hard

A separately excited DC motor (k = 1.2 V·s/rad at full field, R_a = 1.5 Ω, V = 240 V) drives a constant load torque of 7.2 N·m. The field flux is reduced to 75 % of its full value. Find the new speed n.

V = 240 Vk = 1.2 V·s/radR_a = 1.5 ΩT = 7.2 N·mΦ = 75 %

n = rpm

Answer key

1. I_a = 15 A
2. T = 0.6 N·m
3. η = 90.06 %
4. SR = 3.448 %
5. V = 62.12 V
6. n = 2419 rpm

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